1. Field of the Invention
The present invention relates to chromium boride coatings having excellent adhesive wear and corrosion resistance and to a process for preparing such coatings. More particularly, the invention relates to hard, dense, low-porosity, wear and corrosion resistant coatings containing ultrafine chromium boride particles dispersed in a metallic matrix. The invention also relates to a process for preparing such coatings in situ by thermal spray and diffusion reaction techniques.
Throughout the specification, reference will be made to plasma arc spraying and detonation gun (D-Gun) techniques for depositing coatings. Typical detonation gun techniques are disclosed in U.S. Pat. Nos. 2,714,563 and 2,950,867. Plasma arc spraying techniques are disclosed in U.S. Pat. Nos. 2,858,411 and 3,016,447. Other thermal spray techniques are also known, for example, so called "high velocity" plasma and "hypersonic" combustion spray processes, as well as the various flame spray processes. Heat treatment of the coatings is necessary and may be done after deposition in a vacuum or inert gas furnace or by electron beam, laser beam, induction heating, transferred plasma arc or other techniques. Alternative deposition techniques such as slurries, filled fabrics or electrophoresis, followed by heat treatment, are also known. Still other methods include simultaneous deposition and fusion utilizing plasma transferred arc, laser or electron beam surface fusion with or without post deposition heat treatment.
2. Background Art
In the petroleum industry, mechanical gate valves are commonly used for handling a variety of corrosive liquids under high hydraulic pressures. During operation of these valves, the gate is required to move against a valve seat quite rapidly under high mechanical force in order to close and seal the valve. Such conditions create severe adhesive and erosive wear on the metallic surfaces of both the gate and valve seat which can lead to early failure of the valve.
It is common practice in the petroleum industry to employ mechanical gate valves having adhesive and erosive resistant coatings applied to the mating metallic gate and valve seat surfaces. Due to differences in substrate materials and types of wear mechanism involved, the coatings applied to the gate and valve seat surfaces are usually different. For example, a detonation gun tungsten carbide based coating has been used successfully to protect the metallic gate surfaces against adhesive wear while the valve seat has been protected by a Ni-Cr-B-Si-Fe alloy applied as an overlay by known welding techniques.
A problem with these particular coating combinations has been that the valve seat coating is not compatible with many heat treated and hardenable alloys which are useful as substrate materials. For example, a conventional Ni-Cr-B-Si-Fe coating alloy, when applied as an overlay to a valve seat made of AISI 410 stainless steel or AISI 4130 steel usually fails by cracking or spalling after heat treatment. This is due to a mismatch in expansion rates between the substrate and coating. Accordingly, there is a present need to develop new coatings which can be employed with a greater variety of substrate materials.